U.S. patent number 6,388,518 [Application Number 09/818,898] was granted by the patent office on 2002-05-14 for distortion compensation apparatus.
This patent grant is currently assigned to Hitachi Kokusai Electric Inc.. Invention is credited to Tetsuhiko Miyatani.
United States Patent |
6,388,518 |
Miyatani |
May 14, 2002 |
Distortion compensation apparatus
Abstract
A distortion compensation apparatus for compensating for the
distortion occurring in an amplifier with high precision is
provided. Distortion generating devices generate a distortion of
amplitude or phase on a signal to be provided for an amplifier. A
signal level detecting device detects a level of the signal
provided for the amplifier, and a distortion amount control system
controls the amount of distortion generated by the distortion
generating devices on the basis of the level detected by the signal
level detecting device, and at this time, a control timing
adjusting system adjusts the timing for controlling the distortion
amount by a distortion amount control system (D/A converters) so
that the distortion occurring in the amplifier maybe compensated
sufficiently.
Inventors: |
Miyatani; Tetsuhiko (Tokyo,
JP) |
Assignee: |
Hitachi Kokusai Electric Inc.
(Tokyo, JP)
|
Family
ID: |
18748257 |
Appl.
No.: |
09/818,898 |
Filed: |
March 28, 2001 |
Foreign Application Priority Data
|
|
|
|
|
Aug 30, 2000 [JP] |
|
|
2000-260214 |
|
Current U.S.
Class: |
330/149;
330/107 |
Current CPC
Class: |
H03F
1/3247 (20130101); H03F 1/3288 (20130101); H03F
2200/102 (20130101) |
Current International
Class: |
H03F
1/32 (20060101); H03F 001/26 () |
Field of
Search: |
;330/149,136,107 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pascal; Robert
Assistant Examiner: Nguyen; Khanh Van
Attorney, Agent or Firm: Wenderoth, Lind & Ponack,
L.L.P.
Claims
What is claimed is:
1. A distortion compensation apparatus for compensating for
distortion occurring in an amplifier, said distortion compensation
apparatus comprising:
distortion generating means for generating distortion of either
amplitude or phase of a signal provided for the amplifier,
signal level detecting means for detecting a level of the signal
provided for the amplifier,
distortion amount control means for controlling an amount of
distortion generated by said distortion generating means based on
the level detected by said signal level detecting means, and
control timing adjusting means for adjusting a timing for
controlling the amount of distortion by said distortion amount
control means so that the distortion generated in the amplifier may
be compensated, wherein
said distortion generating means comprises a circuit for changing
the amount of distortion generated depending on an analog control
signal,
said distortion amount control means comprises D/A converting means
for converting a digital control signal into the analog control
signal, and issuing the analog control signal at a timing depending
on a timing signal, and the amount of distortion generated by said
distortion generating means is controlled by sending the digital
control signal to said distortion generating means through said D/A
converting means, and
said control timing adjusting means comprises clock signal
generating means for generating a clock signal of a predetermined
period, and timing signal generating means for generating the
timing signal adjusted from a timing of the clock signal generated
by said clock signal generating means, and the timing for
controlling the amount of distortion by said distortion amount
control means is adjusted by sending the timing signal generated by
said timing signal generating means to said D/A converting
means.
2. The distortion compensation apparatus according to claim 1,
wherein said distortion amount control means further comprises
memory means for storing control values in correspondence with
signal levels, and controls the amount of distortion generated by
said distortion generating means by sending a control value
corresponding to the level detected by said signal level detecting
means to said distortion generating means through said D/A
converting means as the digital control signal from said memory
means.
3. The distortion compensation apparatus according to claim 1,
wherein said timing signal generating means comprises a variable
amplifier for amplifying the clock signal generated by said clock
signal generating means by a variable gain, and a limiter for
limiting a level of the clock signal amplified by said variable
amplifier to a predetermined level if the level of the clock signal
amplified by said variable amplifier is more than a predetermined
threshold, and the variable gain of said variable amplifier is
adjusted so that an output signal from said limiter is adjusted
with respect to timing based on level limiting and is issued as the
timing signal.
4. The distortion compensation apparatus according to claim 2,
wherein said timing signal generating means comprises a variable
amplifier for amplifying the clock signal generated by said clock
signal generating means by a variable gain, and a limiter for
limiting a level of the clock signal amplified by said variable
amplifier to a predetermined level if the level of the clock signal
amplified by said variable amplifier is more than a predetermined
threshold, and the variable gain of said variable amplifier is
adjusted so that an output signal from said limiter is adjusted
with respect to timing based on level limiting and is issued as the
timing signal.
5. The distortion compensation apparatus according to claim 1,
wherein said timing signal generating means comprises a comparator
for producing an ON signal by using a variable threshold when a
level of the clock signal generated by said clock signal generating
means is more than the variable threshold, and producing an OFF
signal when the level of the clock signal is less than the variable
threshold, and the variable threshold of said comparator is
adjusted so that an output signal from said comparator adjusted for
on/off timing is issued as the timing signal.
6. The distortion compensation apparatus according to claim 2,
wherein said timing signal generating means comprises a comparator
for producing an ON signal by using a variable threshold when a
level of the clock signal generated by said clock signal generating
means is more than the variable threshold, and producing an OFF
signal when the level of the clock signal is less than the variable
threshold, and the variable threshold of said comparator is
adjusted so that an output signal from said comparator adjusted for
on/off timing is issued as the timing signal.
7. The distortion compensation apparatus according to claim 1,
wherein said timing signal generating means comprises a limiter for
limiting a level of the clock signal to a predetermined level and
issuing an output signal by using a variable threshold if the level
of the clock signal generated by said clock signal generating means
is more than the variable threshold, and the variable threshold of
said limiter is adjusted so that the output signal from said
limiter adjusted with respect to timing based on level limiting and
is issued as the timing signal.
8. The distortion compensation apparatus according to claim 2,
wherein said timing signal generating means comprises of a limiter
for limiting a level of the clock signal to a predetermined level
and issuing an output signal by using a variable threshold if the
level of the clock signal generated by said clock signal generating
means is more than the variable threshold, and the variable
threshold of said limiter is adjusted so that the output signal
from said limiter adjusted with respect to timing based on level
limiting and is issued as the timing signal.
9. The distortion compensation apparatus according to claim 3,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
10. The distortion compensation apparatus according to claim 4,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
11. The distortion compensation apparatus according to claim 5,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
12. The distortion compensation apparatus according to claim 6,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
13. The distortion compensation apparatus according to claim 7,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
14. The distortion compensation apparatus according to claim 8,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal, and the new output signal from said flip-flop is
issued as the timing signal.
15. The distortion compensation apparatus according to claim 3,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
16. The distortion compensation apparatus according to claim 4,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
17. The distortion compensation apparatus according to claim 5,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
18. The distortion compensation apparatus according to claim 6,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
19. The distortion compensation apparatus according to claim 7,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
20. The distortion compensation apparatus according to claim 8,
wherein said timing signal generating means further comprises a
flip-flop for receiving the output signal adjusted of timing, and
issuing a new output signal changed with respect to a duty of the
output signal and another new output signal inverted with respect
to on/off switching of the new output signal, and a selector for
selecting and issuing one of the two new output signals produced by
said flip-flop, and the one of the two new output signals issued
from said selector is issued as the timing signal.
21. A distortion compensation apparatus for compensating for
distortion occurring in an amplifier, said distortion compensation
apparatus comprising:
distortion generating means for generating distortion of either
amplitude or phase of a signal provided for the amplifier,
signal level detecting means for detecting a level of the signal
provided for the amplifier,
distortion amount control means for controlling an amount of
distortion generated by said distortion generating means based on
the level detected by said signal level detecting means, and
control timing adjusting means for adjusting a timing for
controlling the amount of distortion by said distortion amount
control means so that the distortion generated in the amplifier may
be compensated, wherein
said distortion generating means comprises a variable attenuator
for generating amplitude distortion to the signal by varying the
amplitude of the signal provided for the amplifier, and a variable
phase shifter for generating phase distortion to the signal by
varying the phase of the signal provided for the amplifier, said
variable attenuator and said variable phase shifter being connected
in series,
said distortion amount control means controls the amount of
amplitude distortion generated by said variable attenuator by
controlling the amplitude change amount by said variable
attenuator, and controls the amount of phase distortion generated
by said variable phase shifter by controlling the phase change
amount by said variable phase shifter, and
said control timing adjusting means deviates the timing for
controlling the amount of amplitude distortion by said distortion
amount control means and said timing for controlling the amount of
phase distortion by said distortion amount control means, depending
on a lag between the timing of the signal processed by said
variable attenuator and the timing of the signal processed by said
variable phase shifter.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensation
apparatus to compensate for distortion occurring in an amplifier,
and more particularly to a distortion compensation apparatus for
realizing distortion compensation of high precision, by adjusting
the timing for controlling the amount of distortion generated on a
signal to be provided for the amplifier.
2. Description of the Related Art
For example, in a mobile wireless communication system such as
cellular phone system, in order to assure wireless communication
with a mobile station existing at an end of an area (cell) covered
by a base station, and in order to realize wireless transmission of
signals simultaneously to plural mobile stations (plural users)
from the base station depending on the status of communication, the
base station is required to send signals with a large power.
Similarly, in a repeater station (repeating amplifier) for
receiving wireless transmitted signals from the base station,
amplifying these signals, and sending the amplified signals to
mobile stations by wireless transmission, it is also required to
send signals with a large power.
Accordingly, in such a base station or a repeater station, signals
to be transmitted (for example, modulated waves) are amplified to a
desired level by means of a (large) power amplifier (PA) capable of
covering a physical distance up to the end of the cell. In such an
amplifier, however, a nonlinear response (AM-AM conversion or AM-PM
conversion) characteristic may take place near the critical point
(saturation point) of an element, and nonlinear distortion may
occur.
Radio Law demands wireless communication service providers to
strictly regulate their band limit in order to eliminate effects
between different services of wireless communication offered by
using adjacent bands.
As a method of compensating for nonlinear distortion occurring in
the amplifier, hitherto, it was proposed to use a predistorter type
compensation system to compensate for the nonlinear distortion by
generating distortion having a reverse characteristic to the
nonlinear distortion occurring in the amplifier (that is, the
distortion to cancel the nonlinear distortion) in a prior
stage.
Other methods of compensating for nonlinear distortion include, for
example, a feed-forward type distortion compensation system and
negative feedback type distortion compensation system. In the
feed-forward type distortion compensation system, the operation is
advantageously stable, but it is required to extract a distortion
component (occurring in the main amplifier) in the distortion
detecting loop, and to amplify the distortion component with a sub
amplifier in the distortion compensation loop and subtract the
amplified signal from the output signal of the main amplifier.
Hence, there are problems in that the circuit is complicated, and
the power source efficiency is lowered by the sub amplifier. In
contrast, in the predistorter type distortion compensation system,
the structure is relatively simple, and a sub amplifier is not
needed, and it is, hence, advantageous in both circuit scale and
power source efficiency.
An example of a (distortion compensation) amplifying device having
a predistorter for compensating for distortion by such a
predistorter type distortion compensation system is explained
below.
FIG. 11 shows an example of circuitry of an amplifying device with
a predistorter (amplifier with predistortion function), and the
operation of this amplifying device is explained below by referring
to the diagram. This amplifying device is installed in the
transmission section of a base station or repeater station, and the
signal to be transmitted (transmission signal) is entered from a
transmitter. This signal is amplified in the amplifier, and sent
out to an antenna.
First, the signal to be transmitted which is issued from the
transmitter is put into this predistortion circuit, and the signal
is distributed into two, and one distribution signal is fed into
delay means 81, and other distribution signal is fed into a level
detector 85.
FIG. 12 shows an example of spectrum of a signal in an input stage
from the transmitter to the amplifying device, in which the axis of
abscissas denotes the signal frequency [kHz], and the axis of
ordinates indicates the signal level by power ratio [dB]. As shown
in the diagram, at this stage, there is no distortion by a
predistorter (a variable attenuator 82 and a variable phase shifter
83) and no distortion by an amplifier 84, and hence the spectrum
shows a low level of an unnecessary signal out of the band of
use.
The delay means 81 delays the input signal (one distribution
signal), and it sends it to the variable attenuator 82.
The variable attenuator 82 changes (attenuates) the amplitude of
the signal entered from the delay means 81, depending on the
(analog) control signal entered from a D/A converter 88, described
below, to generate an amplitude distortion in an amount
corresponding to the control signal to the input signal, and sends
this signal (including amplitude distortion) to the variable phase
shifter 83.
The variable phase shifter 83 changes the phase of the signal
entered from the variable attenuator 82 depending on the (analog)
control signal entered from a D/A converter 89, described below, to
generate a phase distortion in an amount corresponding to the
control signal to the input signal, and sends this signal
(including phase distortion) to the amplifier 84.
In this example, the predistorter (predistortion generator) is
composed of the variable attenuator 82 and variable phase shifter
83 connected in series and control systems 81, 85, to 90 for
controlling them.
The amplifier 84 amplifies the input signal from the variable phase
shifter 83 to a desired level, and sends out the amplified signal
(from the amplifying device) to the antenna.
FIG. 13 shows an example of a spectrum of a signal issued from the
amplifier 84 when the distortion is not compensated, in which the
axis of abscissas denotes the signal frequency [kHz], and the axis
of ordinates indicates the signal level by power ratio [dB]. As
shown in the diagram, in this case, the spectrum shows there is a
distortion component (leak power to adjacent channels) out of the
band of use due to distortion occurring in the amplifier 84.
Such a distortion component can be compensated for by generating
distortion of a reverse characteristic to the distortion occurring
in the amplifier 84 (amplitude distortion or phase distortion) by
the predistorter (variable attenuator 82 and variable phase shifter
83).
FIG. 14 shows an example of a spectrum of a signal issued from the
amplifier 84 when the distortion is compensated for by the
predistorter, in which the axis of abscissas denotes the signal
frequency [kHz], and the axis of ordinates indicates the signal
level by power ratio [dB]. As shown in the diagram, in this case,
the spectrum shows a decrease of the distortion component (leak
power to adjacent channels) out of the band of use occurring in the
amplifier 84.
The level detector 85 is composed of, for example, an envelope
detector for detecting the envelope of a signal, a low pass filter
(LPF) for extracting a specific frequency component only about the
detected envelope, and an A/D (analog/digital) converter for
converting the detected envelope component from analog to digital
signal. The level detector 85, having such a structure, detects the
level (for example, power level) of the input signal (other
distribution signal), and issues the result of detection to a
controller 90 by a digital value.
Distortion extracting means 86 is composed of, for example, a
directional coupler, and extracts distortion (for example, part of
the amplified signal) included in the amplified signal issued from
the amplifier 84, and sends out to the controller 90.
A clock source 87 generates a clock signal of a specified period,
and issues and supplies the clock signal to the level detector 85
or each processing unit for digital processing such as the two D/A
converters 88, 89 described below.
The D/A(digital/analog) converter 88 converts the digital control
signal entered from the controller 90, described below, into an
analog control signal, according to the timing corresponding to the
clock signal entered from the clock source 87, and issues it to the
variable attenuator 82. This control signal is for controlling the
amplitude change amount (that is, the amount of amplitude
distortion to be generated) in the variable attenuator 82.
The D/A converter 89 converts the digital control signal entered
from the controller 90, described below, into an analog control
signal according to the timing corresponding to the clock signal
entered from the clock source 87, and issues it to the variable
phase shifter 83. This control signal is for controlling the phase
change amount (that is, the amount of phase distortion to be
generated) in the variable phase shifter 83.
The controller 90 is composed of, for example, a digital signal
processor (DSP). On the basis of the detection result (detected
level) entered from the level detector 85, the controller 90 sends
a digital control signal for realizing the amplitude change amount
corresponding to the detection result to the D/A converter 88 from
the variable attenuator 82, and sends the digital control signal
for realizing the phase change amount corresponding to the
detection result to the D/A converter 89 from the variable phase
shifter 83.
More specifically, in the nonlinear characteristic of the amplifier
84, since the level of the output signal is not linear to the level
of the input signal (AM-AM conversion), amplitude distortion
occurs, and since the phase of the output signal is not linear to
the level of the input signal (AM-PM conversion), phase distortion
occurs. The amount of the generated amplitude distortion or phase
distortion varies depending on the level of the signal provided for
the amplifier 84 (the level of the input signal, the level of the
output signal). Accordingly, the controller 90 generates an
amplitude distortion of the amount for canceling the amplitude
distortion generated in the amplifier 84 by the variable attenuator
82, on the basis of the result of detection by the level detector
85 which is the level reflecting the level of the signal provided
for the amplifier 84, and generates the phase distortion of the
amount for canceling the phase distortion occurring in the
amplifier 84 by the variable phase shifter 83.
For example, the correction amplitude distortion characteristic
(the characteristic reverse to the amplitude distortion) for
compensating for the amplitude distortion occurring in the
amplifier 84 and the correction phase distortion characteristic
(the characteristic reverse to the phase distortion) for
compensating for the phase distortion occurring in the amplifier 84
are preliminarily calculated (or measured), and a correction table
storing the control value relating to the amplitude distortion and
the control value relating to the phase distortion corresponding to
each other, for example, with respect to the value of detection
result by the level detector 85 is saved in the memory of the
controller 90. In this case, the controller 90 reads out the
control value relating to the amplitude distortion and the control
value relating to the phase distortion corresponding to the value
of the detection result entered from the level detector 85 from the
correction table, and issues these two control values to the
respective D/A converters 88, 89 as a digital control signal for
controlling the variable attenuator 82 and a digital control signal
for controlling the variable phase shifter 83.
The controller 90 detects the level (for example, power level) of
the distortion component (signal component out of the band of use)
from the signal entered, for example, from the distortion
extracting means 86, and can update the content of the correction
table so that the level to be detected may be smaller (preferably
minimum), that is, the distortion compensation amount may be
larger, thereby enhancing the precision of distortion
compensation.
The delay means 81 has the role of compensating for the time
difference (delay time) of the timing of the one distribution
signal processed by the variable attenuator 82 or variable phase
shifter 83, and the timing of the control signal corresponding to
the level of the other distribution signal entering the variable
attenuator 82 or variable phase shifter 83 from the controller 90
through the D/A converters 88, 89 (ideally the role of matching
these two timings).
That is, when generating distortion(amplitude distortion, phase
distortion) by the variable attenuator 82 or variable phase shifter
83 in a certain signal portion of the input signal, the variable
attenuator 82 or variable phase shifter 83 must be controlled by a
control signal depending on the level of the corresponding signal
portion (not other signal portion), and the delay means 81 is
provided for compensating for the timing of such processing.
However, in the delay means 81 as shown in FIG. 11, for example, it
is disadvantageously hard to adjust the delay time finely
(precisely), and if the delay time becomes long, the precision of
distortion compensation deteriorates. These defects are described
in detail.
That is, in the delay means 81, it is necessary to adjust the delay
time occurring in a physical wiring path, aside from the delay time
taken for the processing of the D/A conversion of the digital
control signal, depending on the detection result, by detecting the
level of the other distribution signal.
As a result of an investigation of, for instance, an amplifying
device assumed by the present inventors (however, this is an
example, and the invention is not limited to this example), for
adjustment of delay time, it is required to adjust in the order of
500 psec (picoseconds or 10.sup.-12 seconds). When this delay time
is adjusted by a semi-rigid cable, a cable of about 10 cm is used.
Generally, it is about 30 to 40 cm from end to end of an electronic
device (circuit) board, and the distance of about 10 cm corresponds
to a delay time easily occurring due to layout of wiring.
Besides, such delay time also varies depending on, for example, the
parasitic capacity of the board, or individual differences of the
devices. In other words, it was hitherto required to adjust the
delay time by controlling the cable length for every device
manufactured (for example, the amplifying device shown in FIG. 11).
Moreover, this delay time varies with, for example, temperature
characteristics of electronic devices, and the delay time is
changed (more or less) when the temperature varies. This delay time
also varies with the duration of use (aging effects).
Thus, such adjustment of delay time is a very important element in
the manufacturing of the device, and it was hitherto difficult to
adjust the delay time in very small time units, and if a difficult
adjustment takes a very long time to perform, the device becomes
very expensive.
The following example shows a result of computer simulation about
the effect of the adjustment error of delay time on the distortion
compensation by a predistorter.
In this example, a single carrier of 5 MHz band is used, and the
condition about the waveform of the signal to be transmitted
conforms to, for example, the specification of 3GPP (3rd Generation
Partnership Project), that is, the number of users is 50, and the
roll-off rate of the filter for limiting the band of the signal is
0.22.
Parameters for investigating the level of distortion component
include third-degree mutual modulation component (IM3),
fifth-degree mutual modulation component (IM5), and others, but in
this example, for the sake of simplicity of explanation, the level
of the distortion component is expressed by the adjacent channel
power ratio (ACPR) [dBc] showing the level of power leaking to a
band adjacent to the band of use.
Specifically, FIG. 15 shows an example of a result of computer
simulation about the effect of the delay time (a relative delay
time of a system for processing one distribution signal and a
system for processing the other distribution signal) on the
correction (compensation) of amplitude distortion, in which the
axis of abscissas denotes the (relative) delay time [.times.2 nsec
(nanoseconds or 10.sup.-9 seconds)] (for instance, graduation 2
indicates 4 nsec), and the axis of ordinates represents the level
of distortion component expressed by the adjacent channel power
ratio (ACPR) [dBc]. In this simulation, the delay time about the
correction of phase distortion (for example, by variable phase
shifter 83) is supposed to be zero.
Moreover, FIG. 16 shows an example of a result of computer
simulation about the effect of the (relative) delay time on the
correction (compensation) of phase distortion, in which the axis of
abscissas denotes the (relative) delay time [.times.2 nsec], and
the axis of ordinates represents the level of distortion component
expressed by the adjacent channel power ratio (ACPR) [dBc]. In this
simulation, the delay time about the correction of amplitude
distortion (for example, by variable phase shifter 83) is supposed
to be zero.
As shown in FIG. 15, the effect of the delay time on the
compensation of amplitude distortion is relatively small, but as
shown in FIG. 16, the effect of delay time on compensation of phase
distortion is relatively large, and the ACPR deteriorates as the
delay time (or its adjustment error) becomes larger.
Herein, the reason why the effect of the delay time is greater on
the compensation of phase distortion than on the compensation of
amplitude distortion is that, generally, the gain variation is
smaller in the amplitude distortion (AM-AM conversion) in the
amplifier, but the gain variation amount is larger in the phase
distortion (AM-PM conversion) in the amplifier. That is, concerning
the phase distortion in the amplifier, since its variation width is
large, the precision of compensation (ACPR, in this case) changes
significantly if the delay time is deviated only slightly.
Numerical values presented as a result of computer simulation shown
in FIG. 15 and FIG. 16 are considered to vary depending, for
example, on the amplifying devices used in the simulation, but the
tendency of the compensation of phase distortion having a larger
effect of delay time as compared with the compensation of amplitude
distortion seems to be the same as the result of this
simulation.
The invention is devised to solve these conventional problems, and
it is, hence, an object thereof to provide a distortion
compensation apparatus capable of, realizing distortion
compensation of high precision by adjusting the delay time finely
(precisely), as mentioned above, and adjusting the timing for
controlling the amount of distortion generated on the signal
provided for the amplifier finely (precisely), when compensating
for the distortion occurring in the amplifier.
SUMMARY OF THE INVENTION
To achieve the object, in the distortion compensation apparatus of
the invention, distortion occurring in the amplifier is compensated
for in the following manner.
That is, in the distortion generating means for generating
distortion of at least one of amplitude and phase on the signal to
be provided for the amplifier, the signal level detecting means
detects the level of the signal provided for the amplifier, and the
distortion amount control means controls the amount of distortion
to be generated by the distortion generating means on the basis of
the level detected by the signal level detecting means, and the
control timing adjusting means adjusts the timing for controlling
the amount of distortion by the distortion amount control means so
that the distortion occurring in the amplifier may be compensated
for sufficiently.
Therefore, by a novel method of adjustment for adjusting the timing
for controlling the amount of distortion generated on the signal
provided for the amplifier, for example, the timing can be adjusted
finely (precisely), so that distortion compensation of a high
precision is realized.
The amplifier, as the object of distortion compensation, is not
particularly limited, and, for example, the amplifier may also be
composed of plural amplifiers. The invention is intended to
compensate for the amplitude distortion or phase distortion
occurring in such an amplifier.
The degree of compensation of distortion occurring in the amplifier
is preferred to be enough to decrease the distortion to zero, but
in the invention, it is not always intended to decrease the
distortion to zero, and it is enough to decrease the distortion
substantially.
To generate distortion by the distortion generating means on the
signal provided for the amplifier, for example, distortion may be
generated on the signal before being amplified by the amplifier, or
distortion may be also generated on the signal after being
amplified by the amplifier.
The distortion generating means is preferred to have both a
function of generating amplitude distortion and a function of
generating phase distortion, but it may also be composed to have a
function of generating amplitude distortion only or a function of
generating phase distortion only.
To detect the level of the signal provided for the amplifier by the
signal level detecting means, for example, the level of the signal
before being amplified by the amplifier maybe detected, or the
level of the signal after being amplified by the amplifier may be
detected.
The level to be detected is not limited, and, for example, the
level of signal amplitude or the level of signal power (usually
proportional to the square of the amplitude) may be detected.
The distortion amount control means controls the amount of
distortion to be generated by the distortion generating means so
that the amount of distortion (distortion of reverse characteristic
to the distortion occurring in the amplifier) generated by the
distortion generating means is enough to cancel the amount of
distortion (amplitude distortion or phase distortion) occurring in
the amplifier. The amount of distortion (amplitude distortion or
phase distortion) occurring in the amplifier may be estimated, for
example, from the level detected by the signal level detecting
means.
The degree of compensation of distortion occurring in the amplifier
by the control timing adjusting means is not particularly defined,
various degrees may be employed as described above, and if the
distortion is not compensated to zero, it is enough, as far as the
distortion compensation is realized, to be at a practically
effective efficiency.
Adjustment of timing for controlling the amount of distortion by
the distortion control means by the control timing adjusting means
corresponds to the adjustment of delay time in the prior art.
The control timing adjusting means is preferred to adjust the
timing (always or periodically) for controlling the amount of
distortion by the distortion amount control means so that the
amount of compensation maybe large, for example, by detecting
always (or, for example, periodically) the amount of compensation
of distortion occurring in the amplifier, but it may also be
designed to set (or fix) the adjustment time preliminarily so that
the distortion occurring in the amplifier may be compensated
substantially.
In the distortion compensation apparatus of the invention,
preferably, the distortion generating means is composed of circuit
(an amplitude changing circuit or a phase changing circuit) for
changing the amount of distortion (amplitude distortion or phase
distortion) occurring depending on the analog control signal
entered from outside (herein, D/A converting means as described
later).
The distortion amount control means is composed of D/A converting
means for converting a digital control signal into an analog
control signal, and issuing it at a timing depending on a timing
signal entered from outside (herein, the control timing adjusting
means), and controls the amount of distortion (amplitude distortion
or phase distortion) generated by the distortion generating means
by sending a digital control signal to the distortion generating
means through this D/A converting means.
The control timing adjusting means is composed of clock signal
generating means for generating a clock signal of a predetermined
period, and timing signal generating means for generating a timing
signal adjusted of timing from the clock signal generated by this
clock signal generating means, and adjusts the timing for
controlling the amount of distortion by the distortion amount
control means by sending a timing signal generated by the timing
signal generating means to the D/A converting means.
In the distortion compensation apparatus of the invention,
preferably, the distortion amount control means further includes
memory means for storing the control value (the control value for
controlling the amount of distortion (amplitude distortion or phase
distortion) generated by the distortion generating means) in
correspondence to the signal level, and controls the amount of
distortion (amplitude distortion or phase distortion) generated by
the distortion generating means by sending the control value
corresponding to the level detected by the signal level detecting
means to the distortion generating means through the D/A converting
means as a digital control signal from the memory means.
In the distortion compensation apparatus of the invention,
preferably, the timing signal generating means is composed of a
variable amplifier for amplifying a clock signal generated by the
clock signal generating means by a variable gain, and a limiter for
limiting the level to a predetermined level and issuing the level
of the signal if the level of the signal provided for the variable
amplifier is more than a predetermined threshold, and adjusts the
gain of the variable amplifier so that the output signal from the
limiter adjusted of timing of level limiting is issued as a timing
signal.
Herein, the predetermined threshold may be any one of various
values depending on, for example, the status of use of the
apparatus. The predetermined level is also not specified, and, for
example, the predetermined threshold (its same level) may be
used.
In the distortion compensation apparatus of the invention,
preferably, the timing signal generating means is composed of a
comparator for producing an ON signal by using a variable threshold
when the level of the clock signal generated by the clock signal
generating means is more than the threshold, and producing an OFF
signal when the level of the clock signal is less than the
threshold, and adjusts the threshold of the comparator so that the
output signal from the comparator adjusted of on/off timing is
issued as a timing signal.
Herein, the predetermined threshold may be any one of various
values depending on, four example, the status of use of the
apparatus.
For example, in the case of a digital signal composed of a value 1
and value 0, the ON signal corresponds to the value 1 signal (or
value 0 signal), and the OFF signal corresponds to value 0 signal
(or value 1 signal).
In the distortion compensation apparatus of the invention,
preferably, the timing signal generating means is composed of a
limiter for limiting the level to a predetermined level and issuing
the level of the signal by using a variable threshold if the level
of the clock signal generated by the clock signal generating means
is more than the threshold, and adjusts the threshold of the
limiter so that the output signal from the limiter adjusted of
timing of level limiting is issued as a timing signal.
Herein, the predetermined threshold may be any one of various
values depending on, for example, the status of use of the
apparatus. The predetermined level may include various levels, and,
for example, the predetermined threshold (its same level) may be
used.
In the distortion compensation apparatus of the invention,
preferably, the duty (the ratio of occupation of ON state in the
signal composed of ON state and OFF state) of the timing signal can
be varied by using a flip-flop. That is, the timing signal
generating means further includes a flip-flop for receiving an
output signal adjusted of timing (output signal from the limiter or
comparator), and issuing a signal changed in the duty of the
signal, and the output signal from the flip-flop is issued as a
timing signal.
In the distortion compensation apparatus of the invention,
preferably, the duty of the timing signal can be changed by using a
flip-flop, and also the timing of the timing signal can be adjusted
(in a wider range) by using a selector. That is, the timing signal
generating means further includes a flip-flop for receiving an
output signal adjusted of timing (output signal from the limiter or
comparator), and issuing a signal changed in the duty of the signal
and a signal inverted in on/off switching of the signal (that is,
the signal changed in the duty of the output signal and inverted in
on/off switching), and a selector for selecting and issuing one of
the two signals produced from the flip-flop, and the output signal
from the selector is issued as a timing signal.
In the distortion compensation apparatus of the invention,
preferably, the distortion generating means is composed of a series
connection of a variable attenuator for generating amplitude
distortion to the signal by varying the amplitude of the signal
provided for the amplifier, and a variable phase shifter for
generating phase distortion to the signal by varying the phase of
the signal provided for the amplifier.
The distortion amount control means controls the amount of
amplitude distortion generated by the variable attenuator by
controlling the amplitude change amount generated by the variable
attenuator, and controls the amount of phase distortion generated
by the variable phase shifter by controlling the phase change
amount generated by the variable phase shifter.
The control timing adjusting means deviates the timing of
controlling the amount of amplitude distortion generated by the
distortion amount control means and the timing for controlling the
amount of phase distortion generated by the distortion amount
control means (for example, by the time corresponding to the
deviation), depending on the lag between the timing of the signal
(the signal provided for the amplifier) processed by the variable
attenuator and the timing of the signal processed by the variable
phase shifter.
Herein, the sequence of the connection of the means for generating
amplitude distortion to the signal provided for the amplifier
(herein, variable attenuator) and the means for generating phase
distortion to the signal (herein, variable phase shifter) is not
specified, that is, amplitude distortion may be generated first and
then the phase distortion later on the signal, or phase distortion
may be generated first and then the amplitude distortion later on
the signal.
The distortion compensation apparatus of the invention is,
preferably, installed in a wireless transmission apparatus for
transmitting signals by wireless means, and compensates for the
distortion occurring in the amplifier for amplifying the signal to
be transmitted by the wireless transmission apparatus. The control
timing adjusting means adjusts the timing for controlling the
amount of distortion by the distortion amount control means, within
an error in a unit of seconds of less than the value of a
reciprocal number of the value of the band of the signal to be
transmitted multiplied by eight (8) (for example, the value of the
carrier frequency interval multiplied by the number of
carriers).
The wireless transmission apparatus may be any apparatus, and
preferably, the base station or repeater station (repeating
amplifier) in a mobile wireless communication system may be used.
The wireless transmission apparatus is not limited to the apparatus
having the wireless transmission function only, and may include an
apparatus having both the wireless transmission function and a
wireless reception function (that is, wireless communication
apparatus).
The value of eight (8) corresponds to the number of over-samplings,
and the value is preferred to be 8 or more as shown above.
More specifically, the error due to timing adjustment is explained
by referring to an example of computer simulation result shown in
FIG. 15 and FIG. 16, as an example of digital predistortion.
Ideally, an optimum distortion compensation (distortion
elimination) is realized when the (relative) delay time shown in
the prior art is zero, but herein, considering an actual apparatus
(causing a certain error), it is investigated if effective
distortion compensation is realized by decreasing the delay time by
an amount by referring to the formulas and examples of computer
simulation result, mentioned above.
Generally, in predistortion, distortion is generated by the
envelope of the signal (the signal to be amplified) entered in the
amplifier. In the amplifier, the nonlinear operation of AM-AM
conversion and AM-PM conversion is carried out by the device, and
it is the cause of distortion. Herein, the AM-AM conversion shows a
phenomenon in which the gain of the amplifier is not constant when
the level of the input signal is large, and the AM-PM conversion
shows a phenomenon in which the phase (output phase) of the signal
issued from the amplifier is changed depending on the level of the
input signal.
Herein, the input signal is a signal to be amplified, and the band
of the signal to be amplified is expressed as shown in formula (1).
The carrier frequency interval is the frequency interval for
detuning adjacent carriers, for example, in the multi-carrier
signal (having plural carrier frequencies). For example, in the
present specification of 3GPP, the carrier frequency interval is 5
MHz. The number of carriers is the number of carriers included in
the multi-carrier signal (differing in frequency). For example, in
the single carrier transmission, the number of carriers =1.
Assuming, for example, a multi-carrier signal of four carriers
(carrier signal=1), in this case, the band width of the signal to
be amplified is 20 MHz (5 MHz.times.4 carriers).
In digital predistortion, predistortion is executed depending on
the fluctuation of the envelope of the input signal. According to
the generally known sampling theorem, in order to follow up the
fluctuation of envelope of input signal accurately, it is required
to execute sampling of 2 times or more of the signal band
width.
That is, the operation period Ts [sec] of the digital circuit
required in this case is expressed in formula (2). As the number of
over-samplings, a numerical value of 2 or more is set.
For example, in the case of a band of signal to be amplified of 5
MHz and 4-times sampling (number of over-samplings=4), Ts is 50
(=1/20 MHz) [nsec]. In this case, as over-sampling, it means to
follow up (to sample) at a speed of 4 times on the envelope
fluctuating at a speed of 5 MHz.
In this case, the maximum value of a relative delay error (delay
time) is 25.0 nsec (half of 50 nsec). That is, in the digital
system, which operates on the clock, for example, by inverting the
on/off state of the clock signal in 50 nsec period composed of ON
state (for example, value 1) and OFF state (for example, value 0),
and selectively using the normal clock signal (non-inverted clock
signal) and inverted clock signal selectively, it is possible to
adjust the delay time in the unit of 25.0 nsec, that is, half value
of the period.
The example of a result of computer simulation shown in FIG. 15 and
FIG. 16 is discussed below. As stated above, since the AM-PM
conversion is more likely to have the effect of time delay, as
compared with the AM-AM conversion, it is discussed herein on the
basis of the example of result of computer simulation about AM-PM
conversion shown in FIG. 16.
Suppose the ACPR required in this system is, for example, -65 dBc
(the standard is -60 dBc and there is a margin of 5 dB). In this
case, referring to FIG. 16, the allowable relative delay error is
about -4 nsec to +2 nsec on the basis of the ideal point (the point
where the relative delay time is zero). That is, the allowable
fluctuation range is 6 nsec, and there is no problem as long as the
delay time is less than this range.
For example, assuming a system of 4-times sampling by using a
single carrier, a delay time T1 adjustable by a clock signal (of
normal rotation only) is expressed in formula (3), and a delay time
T2 adjustable when using also a clock signal of inverse rotation is
expressed in formula (4). ##EQU1##
Discussing the adjustable delay time T2 shown in formula (4), the
time unit for adjusting for realizing ACPR of -65 dBc is within 6
nsec, and hence, it is required to adjust by the digital clock and
adjust the delay time at a precision of 8 times.
Summing them up, to realize ACPR of -65 dBc, a time unit T3 to be
adjusted is shown in formula (5). In the formula, n denotes the
value of the number of over-samplings (4 in this example), and 8 is
the required precision acquired by computer simulation.
##EQU2##
Suppose the ACPR required in the system is about -60 dBc. In this
case, referring to FIG. 16, the allowable relative delay error is
about -8 nsec to +6 nsec, on the basis of the ideal point (the
point where the relative delay time is zero). That is, the
allowable fluctuation range is 14 nsec, so that it is enough to
adjust the delay time within an error of 14 nsec.
Similarly, to realize ACPR of -60 dBc, a time unit T4 to be
adjusted is shown in formula (6). In the formula (6), when the
number of over-samplings is 4, the clock inversion is used, and
further, by adjustment of delay time at a precision of 2 times, it
means that the desired ACPR is achieved. ##EQU3##
Further, suppose that the ACPR required in the system is, for
example, -55 dBc. Herein, ACPR=-55 dBc corresponds, in the 3GPP
standard, to an allowable next adjacent channel leak power ratio
(expressing an allowable leak power to a next adjacent channel).
The next adjacent channel represents a frequency band deviated in
the frequency interval from the adjacent carrier further by one
carrier.
As a specific example, when the frequency of the transmission
signal carrier is 2.1125 GHz and the carrier frequency interval is
5 MHz (=0.005 GHz), the frequency of the adjacent channel is 2.1175
GHz (or 2.1075 GHz), and the frequency of the next adjacent channel
is 2.1225 GHz (or 2.1025 Ghz). That is, it is shifted by each
carrier frequency interval (5 MHz) from the reference carrier to
adjacent channel, and from adjacent channel to next adjacent
channel.
Generally, in actual predistortion, a distortion situation
different from the distortion occurrence situation of a pure
amplifier occurs. That is, in distortion by the amplifier only, the
amount of distortion attenuation attenuates as the frequency
interval from the reference channel becomes wider, from reference
channel to adjacent channel and to next adjacent channel. In
predistortion, on the other hand, for example, the amount of
distortion in the adjacent channel and the amount of distortion in
the next adjacent channel may be nearly the same quantity.
Accordingly, if the leak power standard in, for example, the
adjacent channel is achieved, it is not always guaranteed that the
leak power standard is achieved in the next adjacent channel.
Considering this point, herein, an example of -55 dBc which is the
standard of next adjacent channel leak power is discussed.
In this case, referring to FIG. 16, the allowable relative delay
error is about -13 nsec to +12 nsec on the basis of the ideal point
(the point where the relative delay time is zero). That is, the
allowable fluctuation range is 25 nsec, and it is enough to adjust
the delay time within an error of 25 nsec.
Similarly, to realize ACPR of about -55 dBc, a time unit T5 to be
adjusted is shown in formula (7). In the formula (7), when the
number of over-samplings is 4, by using the clock inversion, (by
adjusting the delay time at a precision of 1 times), it is shown
that the desired ACPR is achieved. ##EQU4##
Thus, in order to obtain the required distortion amount (herein,
ACPR), it is enough to adjust the delay time at least within an
error of 1/(signal band width x number of over-samplings), and
further, in digital signal processing, since all operation timing
is controlled by the clock, it is also possible to adjust the delay
time of half clock, for example, by using the inverted clock.
Further, according to the example of a computer simulation result
shown in FIG. 16, preferably, a design margin for decreasing
distortion to a desired distortion amount should be obtained by
setting the number of over-samplings at a value of 8 or more.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing an example of circuitry of an
amplifying device having a predistorter in a first embodiment of
the invention.
FIG. 2 is a diagram showing an example of circuitry of phase
adjusting means.
FIGS. 3(a)-(d) are diagrams for explaining an example of operation
of phase adjusting means.
FIG. 4 is a diagram showing an example of circuitry of phase
adjusting means.
FIG. 5 is a diagram showing an example of circuitry of phase
adjusting means.
FIGS. 6(a)-(g) are diagrams for explaining an example of operation
of phase adjusting means.
FIG. 7 is a diagram showing an example of circuitry of phase
adjusting means.
FIGS. 8(a)-(g) are diagrams for explaining an example of operation
of phase adjusting means.
FIG. 9 is a diagram showing an example of an image for adjusting
the delay time by perturbation method.
FIG. 10 is a diagram showing an example of circuitry of an
amplifying device having a predistorter in a second embodiment of
the invention.
FIG. 11 is a diagram showing an example of circuitry of an
amplifying device having a predistorter in the prior art.
FIG. 12 is a diagram showing an example of a spectrum of a signal
without distortion.
FIG. 13 is a diagram showing an example of a spectrum of a signal
having distortion.
FIG. 14 is a diagram showing an example of a spectrum of a signal
compensating distortion.
FIG. 15 is a diagram showing an example of a result of computer
simulation about the effect of delay time on correction of
amplitude distortion.
FIG. 16 is a diagram showing an example of a result of computer
simulation about the effect of delay time on correction of phase
distortion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, a(n) (distortion compensation)
amplifying device according to a first embodiment of the invention
is described below.
The amplifying device of this embodiment has a predistorter as an
embodiment of the distortion compensation apparatus of the
invention, and the distortion occurring in the amplifier is
compensated for by the predistorter type distortion compensation
system by using this predistorter.
FIG. 1 shows an example of circuitry of the amplifying device
having the predistorter of the example (an amplifier with
predistortion function). This amplifying device is provided in the
transmission section of a base station or repeater station, for
example, in a mobile wireless communication system, and the signal
to be transmitted (transmission signal) is entered from a
transmitter, and this signal is amplified in the amplifier, and
sent out to an antenna.
As shown in FIG. 1, the amplifying device of the example comprises
delay means 1 for delaying the transmission signal until the
predistortion described below is ready, a variable attenuator 2 for
providing the transmission signal with a correction AM-AM
characteristic for predistortion, a variable phase shifter 3 for
providing the transmission signal with a correction AM-PM
characteristic for predistortion, an amplifier 4 for amplifying the
transmission signal to a predetermined transmission level, a level
detector 5 for detecting the (envelope) level of the transmission
signal, distortion extracting means 6 for extracting a distortion
component signal from the output signal of the amplifier 4, two D/A
converters 7, 8 for converting the digital output signal from a
controller 12 into an analog signal, a clock source 9 for providing
each digital device with a clock, two phase adjusting means 10, 11
for generating clocks of different phases from the clock generated
by the clock source 9, and the controller 12 for adaptively
controlling the predistortion and controlling the phase adjusting
means 10, 11.
An example of operation of the amplifying device of the embodiment
is shown below.
First, the signal to be transmitted issued from the transmitter
enters (in the amplifying device of this embodiment), and this
signal is distributed into two, and one distribution signal is
entered in the delay means 1 and the other distribution signal is
fed into the level detector 5.
The delay means 1 delays the input signal (one distribution
signal), and then sends it to the variable attenuator 2. The delay
means 1 may be composed, for example, by using a delay wire for
delaying a signal, or a band pass filter (BPF).
The variable attenuator 2 changes (attenuates) the amplitude of the
signal entered from the delay means 1, depending on the (analog
voltage) control signal entered from the D/A converter 7, described
below, to generate an amplitude distortion in an amount
corresponding to the control signal to the input signal, and sends
this signal (including amplitude distortion) to the variable phase
shifter 3.
The variable phase shifter 3 changes the phase of the signal
entered from the variable attenuator 2 depending on the (analog
voltage) control signal entered from the D/A converter 8, described
below, to generate a phase distortion in an amount corresponding to
the control signal to the input signal, and sends this signal
(including phase distortion) to the amplifier 4.
In this embodiment, the predistorter is composed of the variable
attenuator 2 and variable phase shifter 3 connected in series and
control systems 1, 5 to 12 for controlling them.
The amplifier 4 is composed of, for example, a (large) power
amplifier, and amplifies the input signal from the variable phase
shifter 3 to a desired (power) level, and sends out the amplified
signal (from the amplifying device of the embodiment) to the
antenna.
In this amplifier 4, for example, when the level of the signal
entered from the variable phase shifter 3 is large, amplitude
distortion or phase distortion occurs. In this embodiment, the
distortion component is compensated for by generating a distortion
of reverse characteristic (amplitude distortion or phase
distortion) to the distortion occurring in the amplifier 4 by the
predistorter (variable attenuator 2 and variable phase shifter
3).
The level detector 5 is composed of, for example, an envelope
detector for detecting the envelope of a signal by using a diode, a
low pass filter (LPF) for extracting a specific frequency component
only about the detected envelope, and an A/D converter for
converting the detected envelope component from an analog signal to
a digital signal. The level detector 5, having such a structure,
detects the level (for example, power level) of the input signal
(the other distribution signal), and issues the result of the
detection to the controller 12 by a digital value (digitized level
information).
The reason for detecting the level of the input signal by the level
detector 5 is, in the predistortion of the present embodiment, to
correct the output level fluctuation or output phase fluctuation
occurring in the amplifier 4 which varies depending on the level of
the input signal (in the amplifier 4), that is, it is necessary to
detect the level of the transmission signal entered in the
amplifier 4 (detected indirectly in this embodiment).
The distortion extracting means 6 is composed of, for example, a
directional coupler, and extracts distortion (for example, part of
the amplified signal) included in the amplified signal issued from
the amplifier 4, and sends the distortion out to the controller
12.
The D/A converter 7 converts the digital control signal entered
from the controller 12, described below, into an analog control
signal, according to the timing corresponding to the clock signal
(timing signal) entered from the phase adjusting means 10,
described below, and issues the analog control signal to the
variable attenuator 2. This control signal is for controlling the
amplitude change amount (that is, the amount of amplitude
distortion to be generated) in the variable attenuator 2.
The D/A converter 8 converts the digital control signal entered
from the controller 12, described below, into an analog control
signal, according to the timing corresponding to the clock signal
(timing signal) entered from the phase adjusting means 11,
described below, and issues the analog control signal to the
variable phase shifter 3. This control signal is for controlling
the phase change amount (that is, the amount of phase distortion to
be generated) in the variable phase shifter 3.
The clock source 9 generates a clock signal of a predetermined
period, and issues and supplies the clock signal to digital
processing parts (excluding two D/A converters 7, 8 in this
embodiment) such as the level detector 5 and the two phase
adjusting means 10, 11, described below.
The phase adjusting means 10 generates a clock signal (timing
shifted clock) shifted in phase from the clock signal entered from
the clock source 9, according to the control from the controller
12, and sends this clock signal (timing signal) to the D/A
converter 7.
The phase adjusting means 11 generates a clock signal (timing
shifted clock) shifted in phase from the clock signal entered from
the clock source 9, according to the control from the controller
12, and sends this clock signal (timing signal) to the D/A
converter 8.
The controller 12 is composed of, for example, a digital signal
processor (DSP). On the basis of the detection result (detected
level) entered from the level detector the controller 12 sends a
digital control signal for realizing the amplitude change amount
corresponding to the detection result to the D/A converter 7 for
the variable attenuator 2, and sends the digital control signal for
realizing the phase change amount corresponding to the detection
result to the D/A converter 8 for the variable phase shifter 3.
More specifically, in the nonlinear characteristic of the amplifier
4, since the level of the output signal is not linear to the level
of the input signal (AM-AM conversion), amplitude distortion
occurs, and since the phase of the output signal is not linear to
the level of the input signal (AM-PM conversion), phase distortion
occurs, and the amount of the generated amplitude distortion or
phase distortion varies depending on the level of the signal
provided for the amplifier 4. Accordingly, the controller 12
generates an amplitude distortion of an amount for canceling the
amplitude distortion generated in the amplifier 4 by the variable
attenuator 2, on the basis of the result of detection by the level
detector 5 which is the level reflecting the level of the signal
provided for the amplifier 4, and generates the phase distortion of
an amount for canceling the phase distortion occurring in the
amplifier 4 by the variable phase shifter 3.
For example, the correction amplitude distortion characteristic
(the characteristic reverse to the amplitude distortion) for
compensating for the amplitude distortion occurring in the
amplifier 4 and the correction phase distortion characteristic (the
characteristic reverse to the phase distortion) for compensating
for the phase distortion occurring in the amplifier 4 are
preliminarily calculated (or measured), and a correction table
storing the control value relating to the amplitude distortion and
the control value relating to the phase distortion corresponding to
each other, for example, with respect to the value of detection
result by the level detector 5 is saved in a memory of the
controller 12. In this case, the controller 12 reads out the
control value relating to the amplitude distortion and the control
value relating to the phase distortion corresponding to the value
(digitized level information) of the detection result entered from
the level detector 5 from the correction table, and issues these
two control values to the respective D/A converters 7, 8 as a
digital control signal for controlling the variable attenuator 2
and a digital control signal for controlling the variable phase
shifter 3.
The controller 12 detects the level (for example, power level) of
the distortion component (signal component out of band of use) from
the signal entered, for example, from the distortion extracting
means 6, and can update the content of the correction table so that
the level to be detected may be smaller (preferably minimum). That
is, the distortion compensation amount may be larger, thereby
enhancing the precision of distortion compensation. Moreover, since
the value to be corrected (the content in the correction table) can
be updated adaptively, it is possible to provide predistortion
capable of coping with a very small error of delay time caused by,
for example, temperature characteristic change or the aging
effect.
Further, the controller 12 controls the phase adjustment (of a
clock signal) performed by the phase adjusting means 10, 11, by
issuing control signals to the phase adjusting means 10, 11. In
this case, the controller 12 of the present embodiment controls the
phase adjustment so that the level of the distortion component
detected from the signal entered from the distortion extracting
means 6 may be smaller (preferably minimum).
Herein, the two phase adjusting means 10, 11 which are
characteristic components of the invention are described in further
detail.
Same as mentioned in the prior art, in this embodiment, the
transmission signal is delayed by the delay means 1 for the
preparation time of predistortion, in the digital region. Ideally,
concerning an arbitrary signal portion for composing the
transmission signal, it is required that the timing of the signal
portion to be entered in the variable attenuator 2 or variable
phase shifter 3 through the delay means 1, and the timing of the
variable attenuator 2 or variable phase shifter 3 controlled by the
controller 12 according to the level of the signal portion should
have the same timing. In this embodiment, since the error of the
delay time by the delay means 1 is relatively large, such a (small)
timing is adjusted by the two phase adjusting means 10, 11.
That is, specifically, a clock is supplied in the digital circuit,
or analog interface such as an A/D converter or a D/A
converter.
For example, supposing a clock signal of 80 MHz is distributed and
supplied into each block from the clock source 9, since the period
of the clock signal is 12.5 nsec, (by the normal clock signal
only), the (relative) delay time can be adjusted only in a unit of
12.5 nsec. Further, by generating an inverse clock signal by using
an inverter, for example, it is possible to control in a unit of
6.25 (=12.5/2) nsec. However, the limit is the delay time
adjustment in the unit of 6.25 nsec.
As discussed in the problems, however, as a result of an
investigation of the amplifying device assumed by the present
inventors (this is only an example, and the invention is not
limited to this example only), as adjustment of delay time in
actual predistortion, it is necessary to adjust the delay time in a
unit of about 500 psec.
Accordingly, the amplifying device of the embodiment includes the
two phase adjusting means 10, 11, and the phase adjusting means 10
generates a clock signal of an independent phase of the clock
signal from the clock source 9. That is, in this embodiment, the
clock signal issued from the clock source 9 is fed into the phase
adjusting means 10, 11, and the phase of the clock signal is finely
(precisely) adjusted in the phase adjusting means 10, 11, and
supplied into each of the D/A converters 7, 8.
In the embodiment, having such a constitution, if the delay time by
the delay means 1 is too short or too long, for example, by
controlling the phase adjusting means 10, 11 by the control signal
from the controller 12, the delay time (control timing of the
variable attenuator 2 or the variable phase shifter 3) can be
corrected finely (precisely) relatively easily.
As a result, predistortion process can be executed correctly at the
timing for processing predistortion on each signal portion
composing the transmission signal, and hence, the leak power out of
the band left over and contained in the output signal of the
amplifier 4 can be sufficiently decreased. Hitherto, a skilled
engineer used to adjust the above-described delay time finely
(precisely) in about a half of a day, but in the amplifying device
of the embodiment, such fine adjustment is easily realized (for
example, automatically by the controller 12).
In the embodiment, as a preferred mode, processing timing is
adjusted independently on each one of the two D/A converters 7, 8.
By contrast, in the configuration for timing adjustment at the time
of A/D conversion (for example, at the time of output of detection
result from the level detector 5) as the input stage of the digital
circuit, it is possible to have timing to be suited to either one
of the two D/A converters 7, 8, but it is not possible to have
(different) timing suited to each one of the two D/A converters 7,
8.
That is, in the constitution of the embodiment, since the delay
time is adjusted (simultaneously) on the two D/A converters 7, 8
provided immediately before the variable attenuator 2 and variable
phase shifter 3, it is possible to adjust the delay time due to
line difference, for example, in the digital circuit, and hence, it
is possible to absorb and eliminate the small (slight) time
difference in the line up to the D/A converters 7, 8.
In the constitution of the invention, meanwhile, since there is a
deviation (offset time) between the timing of the signal issued
from the delay means 1 processed by the variable attenuator 2 and
the timing of this signal processed by the variable phase shifter
3, it is preferred that the phase of the clock signal (timing
signal) given to the D/A converters 7, 8 may be deviated by the
portion corresponding to this deviation.
A specific example of circuitry and an example of operation of the
phase adjusting means 10, 11 are shown below. In the embodiment,
the circuitry of the phase adjusting means 10, 11 is identical, and
the phase adjusting means 10 is representatively explained below in
the following circuitry example.
FIG. 2 shows an example of circuitry of the phase adjusting means
10, and this phase adjusting means 10 is composed of a series
connection of a variable (gain) amplifier 21 and a limiter 22.
The variable amplifier 21 has a characteristic of varying the gain
depending on the control signal (for example, control voltage)
entered from the controller 12, and amplifies the clock signal
entered from the clock source 9 by the gain controlled by the
controller 12, and sends the amplified signal to the limiter
22.
The limiter 22 has a characteristic of issuing the levels of the
signals, for example, when signals having levels over a (fixed)
predetermined threshold are entered, all at the same level (for
example, the level corresponding to the threshold), and limits the
level of the amplified signal entered from the variable amplifier
21 by this characteristic, and sends the limited amplified signal
to the D/A converter 7 (D/A converter 8 in the case of phase
adjusting means 11).
Referring to FIGS. 3(a)-(d), an example of operation of the circuit
shown in FIG. 2 is explained. In the graphs (a) to (d) shown in the
diagram, the axis of abscissas denotes the time and the axis of
ordinates represents the voltage level.
FIG. 3(a) shows an example of waveform of a clock signal issued
from the clock source 9.
FIG. 3(b) shows an example of waveform of a clock signal amplified
by the variable amplifier 21.
In FIG. 3(c), a solid line shows an example of waveform of a signal
after the amplified clock signal is processed by the limiter 22,
and for the convenience of explanation, further, an example of the
waveform of the amplified clock signal is indicated by a dotted
line, together with an example of threshold set by the limiter 22.
As shown in (c), since all input signals of the level above the
threshold of the limiter 22 are clipped at a predetermined level
(ON state), signals of square waves are issued from the limiter
22.
In FIG. 3(d), a solid line shows an example of a waveform of a
signal after processing of the amplified clock signal by the
limiter 22, relating to the case of control of lower gain of the
variable amplifier 21 as compared with the case of FIG. 3(c), for
the convenience of explanation, an example of the waveform of the
amplified clock signal is indicated by a dotted line for reference,
together with an example of a threshold set in the limiter 22 (the
same value as in (c)).
As shown in FIG. 3(d), in the case where the gain of the variable
amplifier 21 is relatively low, when the level of the sinusoidal
wave entered in the limiter 22 becomes relatively large, its level
is clipped. Thus, by varying the gain. of the variable amplifier
21, the width of the time of the ON state of the signal issued from
the limiter 22 is changed depending on the gain.
For example, as indicated by broken line in FIGS. 3(c) and (d),
relating to two cases different in the gain of the variable
amplifier 21, comparing the clipping time of the level of the
amplified signal by the limiter 22, it is known that the phase of
the signal issued from the limiter 22 is deviated depending on the
gain.
In this embodiment, by controlling the gain of the variable
amplifier 21 (that is, the voltage level of amplified clock signal)
by the controller 12, a clock signal (timing signal) shifted in
phase is generated and this signal is used as the input clock of
the D/A converter 7, so that the operation timing of the D/A
converter 7 can be (finely) adjusted. That is, since the D/A
converter 7 operates at the timing of the rise (or fall) of the
clock signal being entered, it therefore, can be adjusted by
slightly (precisely) changing the timing of output of the (analog)
control signal from the D/A converter 7.
Thus, in the constitution of the phase adjusting means 10, 11 as
shown in FIG. 2, by controlling the gain of the variable amplifier
21 from the controller 12, clock signals (timing signals) having
various phases can be generated, and hence, the delay time can be
adjusted finely (precisely).
FIG. 4 shows other example of circuitry of the phase adjusting
means 10, and this phase adjusting means 10 is composed by series
connection of an amplifier 31 and a comparator 32.
The amplifier 31 has a characteristic of being constant. For
example, the (voltage) level of the output signal (for example, the
gain of amplification is constant), and it amplifies the clock
signal entered from the clock source 9, and sends the amplified
signal to the comparator 32.
The amplifier 31 having such a characteristic is generally known as
buffer, and such an amplifier 31 can be omitted from the circuitry
as far as satisfying the condition that phase adjusting means 10 is
located physically close to the clock source 9, or the condition
that the input signal into the phase adjusting means 10 (in this
embodiment, the clock signal from the clock source 9) is not
attenuated.
The comparator 32 receives an amplified signal issued from the
amplifier 31 and a control signal issued from the controller 12,
and compares the level of these two signals, and produces the
result of the comparison as a value 1 (for example, high level) or
a value 0 (for example, low level) to the D/A converter 7.
Specifically, in this embodiment, the (voltage) level of the
control signal entered from the controller 12 to the comparator 32
is used as the threshold in the comparator 32, and this comparator
32 produces an ON signal (signal of value 1 in this case) when the
(voltage) level of the signal issued from the amplifier 31 is
higher than the threshold, and an OFF signal (signal of value 0 in
this case) when the level is less than the threshold.
FIG. 5 shows another example of circuitry of the phase adjusting
means 10, and this phase adjusting means 10 is composed of a series
connection of an amplifier 41 and a limiter 42.
The amplifier 41 has a characteristic of being constant, for
example, the (voltage) level of the output signal (for example, the
gain of amplification is constant), and it amplifies the clock
signal entered from the clock source 9, and sends the amplified
signal to the limiter 42.
Same as described in relation to the circuitry in FIG. 4, such an
amplifier 41 can be omitted from the circuitry as far as satisfying
the specified condition.
The limiter 42 has a characteristic of varying the threshold
depending on the control signal entered from the controller, 12,
and when a signal having a level higher than the threshold is
entered, it also has a characteristic of issuing the level of the
signal all at the same level (for example, the level corresponding
to the threshold), and the level of the amplified signal entered
from the amplifier 41 is limited by this characteristic, and a
controlled amplified signal is sent out to the D/A converter 7.
Specifically, in this embodiment, the threshold of the limiter 42
is changed by the control (voltage) from the controller 12, and the
limiter 42 limits the level when the (voltage) level of the signal
entered from the amplifier 41 is over the threshold, and issues an
ON signal (signal of value 1 in this case) at a specified (voltage)
level.
Referring to FIGS. 6(a)-(g), examples of operation of the circuit
shown in FIG. 4 or FIG. 5 are presented. Herein, the circuit
operation in FIG. 4 and the circuit operation in FIG. 5 have
similar features, and operation examples of these two circuits are
explained together. In FIGS. 6(a) to (g), the lateral direction
denotes the time and the vertical direction indicates the voltage
level of a signal. FIGS. 6(f) and (g) are described in detail
later.
FIG. 6(a) shows an example of waveform of a clock signal issued
from the clock source 9 and entered in the amplifier (amplifier 31
or amplifier 41).
FIGS. 6(b) and (d) show examples of waveforms of a clock signal
amplified (for example, provided with some faxed gain) by the
amplifier (amplifier 31 or amplifier 41).
FIGS. 6(c) and (e) show, by solid line, examples of waveforms of
signals issued from the phase adjusting means 10 (comparator 32 or
limiter 42) to the D/A converter 7 when the threshold (of
comparator 32 or limiter 42) is changed (in FIGS. 6(c) and (d)),
and also show, by dotted lines, examples of waveforms of a clock
signal provided for the amplifier (amplifier 31 or amplifier 41)
for the convenience of explanation, and also examples of the
threshold (set by the comparator 32 or limiter 42) are indicated by
dotted lines.
As shown in FIGS. 6(c) and (e), in the constitution of the phase
adjusting means 10, 11, shown in FIG. 4 and FIG. 5, by controlling
the threshold of the comparator 32 or threshold of the limiter 42
from the controller 12, clock signals (timing signals) having
various phases can be generated, so that the delay time can be
adjusted finely (precisely). That is, in this embodiment, instead
of varying the gain in the amplifier (amplifier 31 or amplifier
41), by varying the value (threshold) of the reference voltage to
the comparator 32 in a later stage or the threshold of the limiter
42 in a later stage, the phase of the clock-signal can be
changed.
FIG. 7 shows a different example of circuitry of the phase
adjusting means 10, and this phase adjusting means 10 is composed
of a series connection of a variable (gain) amplifier 51, a limiter
52, a flip-flop (D-FF) 53, and a selector 54.
In such a circuit structure, the duty of the clock signal (timing
signal) can be controlled by the flip-flop 53. As mentioned above,
the D/A converters 7, 8 operate on the rise or fall of the entered
clock signal (timing signal), but in the actual D/A converter, the
minimum required time (duty) is often specified, for example, in
the high level region or low level region of the input clock. In
this case, in the circuitry shown in FIG. 2, FIG. 4, or FIG. 5,
since the duty fluctuates, the phase variably shifted by the phase
adjusting means 10 may be limited. In this structure, accordingly,
the duty is shaped by the flip-flop 53.
The function and operation of the variable amplifier 51 are, for
example, same as the function and operation of the variable
amplifier 21 shown in FIG. 2, and the variable amplifier 51
amplifies the clock signal entered from the clock source 9 by the
gain controlled by the controller 12, and sends the amplified
signal to the limiter 52.
The function and operation of the limiter 52 are same as the
function and operation of the limiter 22 shown in FIG. 2, and the
limner 52 limits the level of the amplified signal entered from the
variable amplifier 51 by a specified threshold, and sends the
limited amplified signal to the flip-flop 53.
The flip-flop 53 has two input ends and two output ends, and
receives the signal issued from the limiter 52 at one input end,
and receives an inverted signal issued from one output end (output
end of inverted signal) at other input end, and sends out the
inverted signal from one output end to the other input end and to
the selector 54, and also sends a normal rotation signal out from
the other output end to the selector 54.
Herein, the normal rotation signal is, for example, a signal
entered from the limiter 52 to the flip-flop 53 of which the duty
is changed by the flip-flop 53, and the inverted signal is the
signal of which on/off state is inverted.
The flip-flop 53 is generally used for setting the duty at 50%, and
in the construction of this embodiment, the duty of the clock
signal (timing signal) can be set to 50% as shown later in FIGS.
8(a) to (g).
The selector 54 has two input ends and one output end, and also has
a function of changing over the output signal so that (only) the
signal entered from either input end depending on the control
signal from the controller 12 may be selectively issued from the
output end. The selector 54 receives the inverted signal issued
from one output end of the flip-flop 53 at one input end, and
receives the normal rotation signal issued from the other output
end of the flip-flop 53 at the other input end, and sends out
either the inverted signal or normal rotation signal to the D/A
converter 7 according to the control from the controller 12.
Referring now to FIGS. 8(a) to (g), an operation example of the
circuit shown in FIG. 7 is shown. In FIGS. 8(a) to (g), the lateral
direction indicates the time, and the vertical direction represents
the voltage level of a signal.
Specifically, FIG. 8(a) shows an example of waveform of a clock
signal issued from the clock source 9 and entered in the variable
amplifier 51.
FIGS. 8(b) and (d) show examples of waveforms of a clock signal
provided for the variable amplifier 51 by using different gains (in
FIGS. 8(b) and (d)).
FIGS. 8(c) and (e), corresponding to FIGS. 8(b) and (d), show
examples, indicated by solid lines, of waveforms of a signal after
processing of the amplified clock signal by the limiter 52, and an
example, indicated by a dotted line, of a waveform of the amplified
clock signal and an example, indicted by a dotted line, of a
threshold set in the limiter 52, byway of reference.
As shown in FIGS. 8(c) and (e), by varying the gain of the variable
amplifier the phase of the generated clock signal (timing signal)
varies, and the duty of the clock signal is changed at the same
time.
In this embodiment, the duty of the clock signal (timing signal) is
set to 50% by the flip-flop 53.
Specifically, FIG. 8(f) shows an example of waveform of the signal
issued from the limiter 52, and at this stage, the duty of the
signal varies depending on the (voltage) level of the signal issued
from the variable amplifier 51.
In FIG. 8(g), an example of a waveform of a signal issued from the
flip-flop 53 as, for example, normal rotation signal is indicated
by a solid line, and an example of waveform of a signal entered at
one input end of the flip-flop 53 (as shown in FIG. 8(f)) is
indicated by a dotted line for reference. As shown in FIG. 8(g), at
the stage of output of a signal from the flip-flop 53, the duty of
the signal is 50%. In the flip flop 53, for example, signals
repeating on/off states at each rise point of the signal entered
from the limiter 52 are issued, and the duty of such signals is
50%.
In this embodiment, by shifting the rise time (or fall time) of the
clock signal (timing signal) issued from the phase adjusting means
10, 11, the phase of the signal is shifted, but in the circuit
structure up to the flip-flop 53, mentioned above, it is hard to
shift the phase by more than 180 degrees.
Accordingly, in this embodiment, the phase of the clock signal
(timing signal) can be controlled by the selector 54 in a range of,
for example, 360 degrees.
That is, in this embodiment, by using the flip-flop 53 which
generally produces a normal rotation signal and an inverted signal
inverted in the rise time and fall time, mutually, and
specifically, by controlling the selector 54 by the controller 12
so that either one of the normal rotation signal and the inverted
signal may be issued from the selector 54, phase adjustment in a
wide range can be realized.
In the circuitry shown in FIG. 7, since the period of the clock
signal (timing signal) issued from the phase adjusting means 10, 11
is 2 times the period of the clock signal (from the clock source 9)
entered in the phase adjusting means 10, 11 as shown in FIG. 8(g),
the clock source 9 is designed to produce a clock signal having a
speed double that (that is, half of the period) of the specified
rate required in the input clock of, for example, the D/A
converters 7, 8. As a specific example, when the D/A conversion
rate by the D/A converters 7, 8 is 80 MHz, a clock source 9 for
producing a clock signal of 160 MHz is used.
In FIG. 7, the flip-flop 53 and selector 54 are added to circuitry
that is the same as shown in FIG. 2, and similarly, it is possible
to adjust the duty of the clock signal (timing signal) by
installing a flip-flop in a later stage of the comparator 32 shown
in FIG. 4 and in a later stage of the limiter 42 shown in FIG. 5.
Further, by installing a selector in a later stage of the
flip-flop, it is also possible to expand the adjustable phase range
in relation to the clock signal.
FIG. 6(f) shows an example of a waveform of a signal issued from
the comparator 32 or limiter 42, the same as is shown, for example,
in FIG. 8(f), and FIG. 6(g) also shows, the same as is shown, for
example, in FIG. 8(g), an example of a waveform of a signal
converted by 50% in the duty of the signal by the flip-flop as
indicated by a solid line, and an example of a waveform (in FIG.
6(f)) of the signal before conversion as indicated by a dotted
line.
Referring next to FIG. 9, the perturbation method is shown as an
example of a manner of adjusting the (relative) delay time by
controlling the phase adjusting means 10, 11 by the controller
12.
That is, the distortion compensation by predistortion of the
embodiment is conducted in conformity with the envelope of the
signal to be amplified (by the amplifier 4). Thus, when
compensating for the distortion of the signal to be amplified
according to the measured envelope, it is necessary to adjust the
(relative) delay time between the control system of distortion
compensation (the system of the controller 12 or D/A converters 7,
8 side) and the main signal system in which the signal to be
amplified flows (the system of the variable attenuator 2 or
variable phase shifter 3 side).
Supposing that the adjustment of delay time is imperfect, a
distortion component may be left over due to its effect in the
signal issued from the amplifier 4 provided in a final stage. The
presence or absence of such a distortion component can be easily
judged by using a spectrum analyzer or the like, and the controller
12 in the embodiment detects the residual amount of distortion due
to presence of delay time by monitoring the amount of distortion
component by using the distortion extracting means 6.
As an example of the adjusting manner of the delay time as
mentioned above, the adjustment by perturbation method is explained
below.
Specifically, in the perturbation method, the delay time is
adjusted by repeating the following process in (1) to (4).
(1) The amount of the distortion component is saved as P0 in the
delay time T at the present moment (after the process in (4) below,
T=T').
(2) The delay time is adjusted by (+.tau.) (that is, the delay time
is further delayed to (T+.tau.)), and the amount of the distortion
component at this time is saved as P+.
(3) Together with step (2), the delay time is adjusted by (-.tau.)
(that is, the delay time is advanced to (T-.tau.)), and the amount
of the distortion component at this time is saved as P-.
(4) The distortion amounts P+ and P- are compared, and the delay
time corresponding to the smaller distortion amount (either P+ or
P-) is saved as a new delay time T' (T'=T+.tau., or
T'=T-.tau.).
By repeating the process in (1) to (4), the updated delay time T
gets closer to the optimum state, and the difference between the
distortion amount P0 and the distortion amount P+ or P- becomes
smaller.
Or, for example, every time the difference between the distortion
amount P0 and the distortion amount P+ or P- becomes smaller,
preferably, the adjusting time used at step (2) or step (3) may be
changed from (.+-.t) to (.+-.t/2) to reduce the adjusting time by
half, so that the delay time can be adjusted adaptively.
FIG. 9 shows an example of an image of adjusting the delay time by
the perturbation method, in which the axis of abscissas denotes the
(relative) delay time, the axis of ordinates in the upper direction
represents the amount of the distortion component, and the axis of
ordinates in the lower direction shows the lapse of time. The
diagram also includes a curve P showing the amount of the
distortion component (the distortion component extracted by the
distortion extracting means 6) caused by the residual delay time,
and the delay time becomes zero at a position of minimum distortion
component amount.
More specifically, in the perturbation method, first, the position
of the delay time T at the present moment is set as the start point
(point 1 in the diagram).
Consequently, at the next lapse of time, the distortion amount at
position of delay time (T+.tau.) and the distortion amount at
position of delay time (T-.tau.) are compared, and the delay time
corresponding to the smaller distortion amount, for example,
(T+.tau.) is selected as an updated delay time T' (point 2).
Further, at the next lapse of time, the distortion amount at
position of delay time (T+.tau.+.tau.) and the distortion amount at
position of delay time (T+.tau.-.tau.) are compared, and the delay
time corresponding to the smaller distortion amount, for example,
(T+.tau.+.tau.) is selected as an updated delay time T' (point
3).
As the updated delay time is getting closer to the optimum delay
time (zero), it is judged that the distortion component amount is
converging, and at the next lapse of time, the distortion amount at
the position of the delay time (T+.tau.+.tau.-.tau./2) and the
distortion amount at the position of the delay time
(T+.tau.+.tau.-.tau./2) are compared, and the delay time
corresponding to the smaller distortion amount, for example,
(T+.tau.+.tau.-.tau./2) is selected as an updated delay time T'
(point 4).
By repeating such a process, the updated delay time is gradually
set closer to the optimum delay time (zero).
As mentioned above, preferably, the delay time should be adjusted
by a time error within, for example, [1/{ carrier frequency
interval.times.number of carriers.times.n}] (that is, in this
embodiment, the difference from the optimum delay time of zero).
Herein, as the carrier frequency interval and the number of
carriers, the values relating to the transmission signal to be
processed by the amplifying device of the embodiment are used, and
the value of n is a positive number of 8 or more. The product of
(carrier frequency interval.times.number of carriers) corresponds
to the band of the transmission signal.
Thus, in the predistorter provided in the amplifying device of the
embodiment, by adjusting the timing for controlling the amount of
distortion (amplitude distortion or phase distortion) generated by
the variable attenuator 2 or variable phase shifter 3 on the
transmission signal to be provided for the amplifier 4 by the phase
adjusting means 10, 11, the timing can be adjusted finely
(precisely), so that distortion compensation of high precision can
be realized.
Specifically, the failure of normal predistortion due to the
presence of slight time delay can be avoided. Moreover, the
adjustment process of slight delay time requiring a long time in
the prior art can be done in a short time, and thereby the cost of
the apparatus can be reduced.
In this embodiment, the amplifier 4 corresponds to the amplifier
(as the object of distortion compensation) of the invention.
In the embodiment, the distortion generating means of the invention
is composed by the function of the variable attenuator 2 and
variable phase shifter 3. In the embodiment, the variable
attenuator 2 and variable phase shifter 3 compose a circuit capable
of changing the amount of distortion to be generated depending on
the analog control signal entered from outside.
In the embodiment, the signal level detecting means of the
invention is realized by the function of the level detector 5.
In the embodiment, the distortion amount control means of the
invention is realized by the function of the controller 12 and two
D/A converters 7, 8.
In the embodiment, the control timing adjusting means of the
invention is realized by the function of the clock source 9 and two
phase adjusting means 10, 11.
In the embodiment, the D/A converting means of the invention is
realized by the function of the two D/A converters 7, 8.
In the embodiment, the clock signal generating means of the
invention is realized by the function of the clock source 9.
In the embodiment, the timing signal generating means of the
invention is realized by the function of the two phase adjusting
means 10, 11 for generating a clock signal (timing signal) adjusted
in its timing.
A (distortion compensation) amplifying device according to a second
embodiment of the invention is explained below by referring to FIG.
10.
The amplifying device of the embodiment has a predistorter which is
an example of the distortion compensation apparatus of the
invention, and the distortion occurring in the amplifier is
compensated by the predistorter type distortion compensation system
by using this predistorter.
FIG. 10 shows an example of circuitry of an amplifying device
having the predistorter of the embodiment (an amplifier with
predistortion function). This amplifying device is provided in the
transmission section of abase station or repeater station, for
example, in a mobile wireless communication system, and the signal
to be transmitted (transmission signal) is entered from a
transmitter, and this signal is amplified in the amplifier, and
sent out to an antenna.
As shown in the diagram, the amplifying device of the embodiment
comprises delay means 61, a variable attenuator 62, a variable
phase shifter 63, an amplifier 64, a level detector 65, distortion
extracting means 66, two memories 67, 68, two D/A converters 69,
70, a clock source 71, two phase adjusting means 72, 73, and a
controller 74.
The composition of the amplifying device of the embodiment is
similar to the composition of the amplifying device of the first
embodiment shown in FIG. 1, except that the memories 67, 68 for
controlling the variable attenuator 62 and variable phase shifter
63 are provided outside of the controller 74. In the following
explanation, same parts as in the amplifying device of first
embodiment shown in FIG. 1 are not specifically described, and
different parts as in the amplifying device of the first embodiment
are described in detail.
That is, in the amplifying device of the embodiment, as the
structure for controlling the variable attenuator 62, the memory 67
is provided between the controller 74 and the D/A converter 69, and
as the structure for controlling the variable phase shifter 63, the
memory 68 is provided between the controller 74 and the D/A
converter 70. In the amplifying device of the embodiment, the
result of detecting the signal level by the level detector 65 is
issued to the two memories 67, 68 as a digital value.
One memory 67 has a look-up table (LUT) for controlling the
variable attenuator 62, and this look-up table stores the (digital)
control values for controlling the amplitude distortion to be
generated in the variable attenuator 62, in correspondence to the
digital values (of detection result) issued from the level detector
65. Herein, as the control value, the value for realizing the
amplitude distortion (the amplitude distortion of reverse
characteristic of the occurring amplitude distortion) capable of
compensating for the amplitude distortion occurring in the
amplifier 64 by predistortion when the digital value (detection
result) corresponding to the control value is obtained is, for
example, preliminarily calculated (or measured) and stored.
Similarly, the other memory 68 has a look-up table (LUT) for
controlling the variable phase shifter 63, and this look-up table
stores the (digital) control values for controlling the phase
distortion to be generated in the variable phase shifter 63, in
correspondence to the digital values (of detection result) issued
from the level detector 65. Herein, as the control value, the value
for realizing the phase distortion (the phase distortion of reverse
characteristic of the occurring phase distortion) capable of
compensating for the phase distortion occurring in the amplifier 64
by predistortion when the digital value (detection result)
corresponding to the control value is obtained is, for example,
preliminarily calculated (or measured) and stored.
Thus, the memory 67 has a look-up table for storing the corrected
AM-AM characteristic, and the other memory 68 has a look-up table
for storing the corrected AM-PM characteristic. These look-up
tables execute the process of issuing the stored value (control
value) corresponding to the input address to the D/A converters 69,
70 respectively, using the digital value (control value) entered
from the level detector 65 as the address, and in this embodiment,
by such processing, the amount of distortion generated by the
variable attenuator 62 and variable phase shifter 63 can be
controlled.
The D/A converters 69, 70 of the embodiment convert the digital
control signals entered from the memories 67, 68 into analog
control signals, and send the analog control signals to the
variable attenuator 62 and variable phase shifter 63.
In this embodiment, meanwhile, on the basis of the level of the
distortion component detected by, for example, the extracting means
66, the stored contents in the memories 67, 68 are adaptively
rewritten by the controller 74, and such a constitution can realize
predistortion capable of processing an error of a small delay time
occurring due to, for example, temperature characteristic changes
and aging effects.
Thus, in the amplifying device of the embodiment, since the
memories 67, 68 for controlling the variable attenuator 62 and
variable phase shifter 63 are composed separately from the
controller 74 and disposed outside of the controller 74, fast
access to the memories 67, 68 is possible, so that the efficiency
of processing can be enhanced.
In the embodiment, the memory means of the invention is realized by
the function of the two memories 67, 68.
For example, in the amplifying device of the embodiment, the
digital section operating at 80 MHz is assumed, but generally if
the D/A conversion process at about 80 MHz (12.5 nsec) is
necessary, it is often difficult to access directly from the
controller 74. The reason is because the controller 74 is often
composed of a digital signal processor (DSP), and in the digital
signal processor, 30 Hz is about the limit for the input and output
rate (access speed of external hardware).
In contrast, in the amplifying device of the embodiment, as
mentioned above, fast access is possible by disposing the memories
67, 68 outside of the controller 74, and in the case of processing
a transmission signal of a wide band, for example, fine (precise)
phase adjustment (timing adjustment of D/A conversion) can be
realized easily.
The constitution of the distortion compensation apparatus according
to the invention is not limited to the illustrated examples alone,
but may be changed and modified in various forms.
For example, the sequence of disposition of the variable attenuator
and variable phase shifter for predistortion is arbitrary. Also,
for example, the structure for acquiring the signal (error signal)
of distortion component from the output signal of the amplifier (as
the object of distortion compensation), and the structure of the
level detector may be realized in various modes.
For example, although not shown in the foregoing embodiments, a low
pass filter (LPF) may be provided between the D/A converter and
variable attenuator, or between the D/A converter and variable
phase shifter, and the output signals from these D/A converters may
be smoothed by the low pass filter. The presence or absence of such
a low pass filter may be arbitrarily set, depending on the
situation of use of the device or the like.
The application field of the distortion compensation apparatus of
the invention is not limited to the shown fields alone, but the
invention includes various other fields.
For example, the distortion compensation apparatus of the invention
may be applied in the apparatus employing the predistortion type
distortion compensation system using digital processing, and it is
also possible to apply in the apparatus employing the predistortion
type distortion compensation system using analog processing.
As various processes of a control program executed as the
distortion compensation apparatus of the invention, for example, in
a hardware resource having a processor and memories, the invention
may be designed to control by the processor which executes the
control program stored in a ROM, or various function means for
executing the processes may be composed as independent hardware
circuits.
The invention also relates to a computer-readable recording media,
such as a floppy disk and CD-ROM storing these control programs,
and by feeding the control programs into a computer from the
recording media, and executing by a processor, the process of the
invention can be executed.
As explained herein, according to the compensation distortion
apparatus of the invention, for example, when controlling the
amount of amplitude distortion or phase distortion generated by the
variable attenuator or variable phase shifter on the signal to be
provided for the amplifier, on the basis of the detection result of
the level of the signal provided for the amplifier, the control
timing is adjusted so that the distortion occurring in the
amplitude may be compensated for sufficiently, and it is possible
to adjust the timing finely (precisely), so that distortion
compensation of high precision is realized.
* * * * *